1. Technical Field
This invention generally relates to current sources, and more specifically relates to low noise current sources with variability and selectable slew rates used for magnetoresistive (MR) heads.
2. Background Art
In magnetic technology, a magnetoresistive (MR) read element requires a DC current bias to excite the element. Various techniques are used to bias and sense the change in resistance of the MR head.
One method of generating the DC current bias is achieved by using a low pass feedback loop that maintains a constant DC voltage across the head. The output of the feedback loop is a current source that maintains a constant voltage drop across the MR element. A single stripe MR head requires two connections. One connection is from the current source in the voltage bias control loop. The other connection of the head could be a DC voltage, ground, or another current source. If another current source is used, it is normally regulated by a feedback loop that regulates the common mode voltage of the head to a desired voltage.
Typical low corner frequencies of the voltage bias feedback loop are on the order of 100 KHz to 1 MHz, consequently a low frequency pole is required. The change of resistance of an MR head is very small, generating typically less than 1 mV of sign The small signal requires the bias current through the MR to be low noise.
U.S. Pat. No. 4,879,610, entitled "Protective Circuit for a Magnetoresistive Element," by Jove et al., issued Nov. 7, 1989, and assigned to International Business Machines Corporation, discloses a typical current source circuit design for biasing MR heads. A capacitor is used to filter out noise, and is charged and discharged according to a feedback circuit.
A typical disk drive has two MR heads for each disk or platter. Since each MR head has a different resistance, each MR head may require a different voltage bias for optimum operation. It is important then that the voltage drop across the MR be variable from head to head. As a result, the current or voltage bias must also change, a feature not disclosed in typical MR voltage bias feedback loop circuits.
Furthermore, as the currents through the read head increase, it is difficult to keep the input impedance of the current source high enough to allow noise filtering without the use of a large capacitor. Unfortunately, the slew rate of the current source circuit is limited by how fast this capacitor can be charged, and in a closed loop system where the current may be different for different heads, head switching times are limited in part by the charging rate of the capacitor.
Some other examples of prior art circuitry that affect slew rate and/or show related circuitry are found in the following U.S. Patents and article and are hereby incorporated by reference: U.S. Pat. No. 4,390,825 issued to Ginn in June 1983; U.S. Pat. No. 4,498,058 issued to Benrud in February 1985; U.S. Pat. No. 4,540,952 issued to Williams in September 1985; U.S. Pat. No. 4,706,138 issued to Jove et al. in November 1987; U.S. Pat. No. 4,881,045 issued to Dillman in November 1989; U.S. Pat. No. 4,902,984 issued to Vinn et al. in February 1990; U.S. Pat. No. 4,992,674 issued to Smith in February 1991; U.S. Pat. No. 5,008,565 issued to Taylor in April 1991; U.S. Pat. No. 5,153,452 issued to Iwamura et al. in October 1992; U.S. Pat. No. 5,160,857 issued to Barre in November 1992; U.S. Pat. No. 5,317,669 issued to Anderson et al. in May 1994; U.S. Pat. No. 5,343,164 issued to Holmdahl in August 1994; U.S. Pat. No. 5,384,501 issued to Koyama et al. in January 1995; and "Magnetic Recording Channel Front-Ends" by Klaassen, IEEE Transactions on Magnetics, Vol. 27, No. 6, November 1991.
Although the aforementioned patents describe methods and boost circuits for enhancing the slew rate of a system and other similar features, these boost circuits would normally not work very well for MR current source circuitry since they would change the bias levels of upstream circuitry to reduce the settling time. Settling times of changes of the current source are shorter than what the normal bandpass used for noise filtering and/or the voltage bias control loop. Shorter settling times are required when initially biasing an MR element during head to head switching. Attempting to decrease settling time by increasing the biasing of an upstream amplifier could lead to differences in the current source value in the `fast settle` mode when compared to the bias level achieved in the normal mode of operation. If the difference is large enough, the circuitry must resettle at normal bias levels during normal mode operations, thus ultimately slowing up the system.
Therefore, there existed a need to provide a current source circuit and boost circuit utilizing technology that would not only allow for variability and a faster head switching time, but would maintain and may even increase stability and noise reduction.